In respect of replicating microstructures on plastic elements produced in a machine of the kind defined in the introduction, it is known to produce first an original master in some suitable way, and then to produce a matrix for use in said machine on the basis of this master. Matrices of this kind can be produced by coating a master or an original that has a positive microstructure on one surface with a metal layer or a metallic coating and removing the negative-microstructured metal layer from the master to thereby obtain a metal plate that can serve as a matrix in the compression moulding, embossing and/or injection moulding press. Normally each mould half can have its own matrix and a flowing, hot (approximately 400° C.) plastic mass is pressed under high pressure into a delimited mould cavity formed by cavities in brought together mould halves The flowing hot plastic mass is then allowed to solidify (at approximately 140° C.) between the brought together mould halves before the mould halves are opened and the solidified element can be pressed out.
Lithographic processes, in particular lithographic processes that have been developed primarily for use in the micro-electrical field, are an example of known methods for producing a master. One of these methods is based on etching a semiconductor surface and/or depositing material thereon. Other methods are based on the removal of parts of material with the aid of a laser, so-called laser ablation, with the aid of traditional NC-machines, with the aid of precision-controlled, high-speed diamond millers, with the aid of electric discharge machining (EDM), wire EDM and/or some other suitable method.
Such originals or masters are normally produced from a material that is chosen to be suitable with respect to a given machining process.
In the case of lithographic processes, the material is most often a sheet of silicon, glass or quartz, whereas in the case of laser ablation the material most often used is a sheet of plastic composite and/or a polymeric material.
In the case of metal processing methods, plastics and soft metals may both be suitable.
It is well known that the requirements of a given replication process on a given material in the matrix and the plastic element are not the same as the requirements that must be met with respect to the original or the master. For instance, with respect to injection moulding of such plastic elements where one or more surface parts shall present a microstructure, one or both of the mould halves of the machine and the matrix used therein must be made of a stable material that can withstand the high pressures that occur during the course of manufacture, and which will not be worn down unnecessarily quickly by the thermal and mechanical wear-and-tear to which the mould halves and the matrix are subjected during the casting or moulding process.
It is known to produce such matrices, and primarily matrices for use with microstructure, by transferring the shape and surface structure of a master to a metal plate which can then serve as a matrix.
One manufacturing method is based on first producing a master on a surface of a glass plate, a semiconductor plate or a metal plate, coating the surface with a light-sensitive layer and exposing selected surface sections of this light-sensitive layer through the medium of a laser or the like, and washing and cleaning the selected surface sections. A metal layer is applied to the exposed and cleaned surface of the master, through the medium of a sputtering process, a vapour deposition process, and/or through the medium of a plating or cladding process, for the length of time required to form a metal plate. The metal plate can then be removed from the master. The metal plate has a first surface which exhibits a negative microstructure which is intended to face towards the inside of a mould cavity. The metal plate can be used as a matrix after further machining, i.e. smoothing, of a second surface that faces towards the mould half in the machine.
It is this method that is presently used in the manufacture of a matrix used in an injection moulding press for the production of optical discs, e.g. CD discs.
Other ways of producing a matrix or a master include: an electrically insulating microstructured disc serving as a master or matrix can be coated with a thin metal layer by means of a sputtering process and/or by vapour deposition; an electrically conductive microstructured disc or layer that functions as a master or matrix can be coated with a much thicker metal layer by means of a plating or cladding process; a disc intended to function as a matrix can be coated with a thin electrically conductive layer, such as a nickel, silver, or gold layer or some like metal layer, by means of a plating or cladding process.
It is also known to connect a metal layer electrically and to submerge a disc in a solution that comprises among other things, metal ions, and to pass an electric current through the solution onto the disc or master unit and therewith cause metal ions to precipitate as pure metal onto the surface of the disc. In this way a structure can be produced in metal that has the inverse function of the microstructure on the master.
It has been found that the above method can be readily applied in respect of flatter structures, particularly when the depth of the microstructure is limited to, or smaller than, about 0.2 μm.
It has been found that in forming a matrix the metal build-up on the microstructure-carrying surface of a master results in minor defects or irregularities on the rear side of the matrix, which irregularities are caused by the microstructure, and that it is necessary to subsequently smooth said rear side in order for it to lie in effective abutment, e.g. flat, on a flat surface on the mould half that supports it in the pressing machine used.
Practical applications have shown that in the case of deeper structures in the master microstructure, the master pattern will be embossed on the rear side of the matrix or metal plate.
Various procedures are known for reducing or eliminating this problem.
A first measure is to apply an extremely thick layer of metal by means of a plating process or some equivalent process. The resulting plate which is intended to serve as the matrix will be strong and stable. The plate can then be placed in equipment in which the metallic rear side of the plate can be smoothed down or levelled mechanically, such as by a grinding, polishing and/or lapping process, while still retaining sufficient strength to serve as a matrix.
The process of applying a truly thick layer of metal as in the case of deeper microstructures takes a relatively long time to achieve, for instance it will take from 10 to 20 hours to apply a nickel cladding which is a few millimetres thick.
Furthermore, it takes considerable time to grind and/or polish down the metallic rear side to a smooth surface. Moreover, the adhesion between the master microstructure and the conductive metal layer in the matrix must be capable of withstanding the tensions that are generated in the interface therebetween.
The use of available grinding and/or polishing equipment for smoothing the metallic rear side to a flat surface also requires the master to be very stable.
Various methods are known to counteract the problem arising from a defective or irregular and uneven rear side, by applying different plating processes so as to be able to level out the growth of the metal layer against a planar metallic rear side.
One known method in this respect uses a pulsed field instead of a direct current with constant field. However, in principle, a metallic layer takes longer to grow with a pulsed field than with a direct current. Using suitably adapted parameters and chemical compositions, this method enables the deep microstructure parts to be coated and built-up more quickly than the shallower microstructure parts, meaning that the deep structures will be overgrown and the metallic rear side will become relatively flat.
Practical experience has shown, however, that the metallic rear side must still be smoothed down, by grinding, polishing and/or lapping said surface.
With respect to the tine consumption of the two methods involving a pulsed field and a constant field, the coating time in the first method will be longer than the coating time in the latter method, whereas the time taken to smooth down said surface will be shorter in the first method than in the second method.
When manufacturing plastic elements with a positive surface related microstructure, different means and arrangements are known for supplying the matrices belonging to the mould halves, and consequently the matrices in general, with alternating heating and cooling.
Heat is applied in order to thereby make the composite plastic or the plastic material used more easily flowing against the surface of the matrix in order to in this way be able to improve the replicating of the microstructure.
It is also known to apply cooling, such as a cold fluid, in the form of oil or water, or gas, in the form of air, to the mould halves of the matrix in order to thereby, immediately after the finishing of the manufacturing process in the machine, to, via the matrix, cool the plastic element down to the solidification temperature so that the positive surface structure belonging to the plastic element remains intact.
It is herewith obvious that because of the matrices' and the mould halves' large heat storing capacity, large heat and cooling transport system are required, which lead to a consequently slow manufacturing speed.
Since the manufacturing speed is extremely dependent on the time taken to heat up the matrix surface, by a heat supply to the mould halves, during the injection process and the time for a subsequent cooling of the cast element via the matrix surface and the mould halves, different measures have been suggested.
Thus, it has been suggested to form channels in the mould halves and supply hot water respectively cold water through these, but because of the high pressure which exists inside the mould cavity it is technically difficult to position them optimally close to the matrix.
With the intention of further reducing the cycle time it is previously known to use a heat insulating layer between the matrix and the mould halves (see—Optimizing Pit Replication Through Managed Heat Transfer—by Thomas Hovatter, Matthew Niemeyer and James Gallo, published by GE Plastics, Pittsfield, Mass. USA).